In general, in a hybrid excavator, a boom cylinder or the like is expanded and contracted by a hydraulic fluid discharged from a hybrid actuator (e.g., hydraulic pump-motor) in response to the drive of an electric motor to cause a work apparatus, i.e., an attachment such as a boom or the like to be manipulated. In other words, as the electric motor is rotated in a forward and reverse direction, the expansion and contraction of the boom cylinder can be controlled. In a work mode in which the boom descends, a high pressure is generated in a large chamber of the boom cylinder by the boom's own weight, and the hydraulic pump-motor is driven by a hydraulic fluid discharged from the large chamber to cause the electric motor to generate electricity.
A general hybrid excavator shown in FIGS. 1 to 5 includes:
an electric motor 11;
a hydraulic pump-motor 12 that is connected to the electric motor 11 and is driven in a forward or reverse direction;
a hydraulic cylinder 15 (e.g., not limited to a boom cylinder) that is expanded and contracted by a hydraulic fluid that is supplied along first and second flow paths 13 and 14 connected to the hydraulic pump-motor 12;
first and second hydraulic valves 16 and 17 that are installed in the first and second flow paths 13 and 14 between the hydraulic pump-motor 12 and the hydraulic cylinder 15, respectively, and are shifted to control the first and second flow paths 13 and 14 in response to a control signal applied thereto from the outside; and
a third hydraulic valve 21 (shifted using a pressure of the first and second flow paths 13 and 14 as a pilot signal pressure) that is installed in a connection path 20 connected to first and second branch flow paths 18 and 19 that are branch-connected to the first and second flow paths 13a and 14a on an upstream side of the first and second hydraulic valves 16 and 17 and the first and second flow paths 13b and 14b on a downstream side of the first and second hydraulic valves 16 and 17, respectively, and compensates for or bypasses a flow rate of the hydraulic fluid in order to overcome a difference in flow rate of the hydraulic fluid, which occurs due to a difference in cross section between a large chamber 15b and a small chamber 15a of the hydraulic cylinder 15 when the hydraulic pump-motor 12 is rotated in a forward and reverse direction.
In this case, the configuration of an attachment 6 consisting of a boom 1, an arm 2, and a bucket 3, which are driven by respective hydraulic cylinders 15, 4 and 5, and an operator's cab 7 is the same as that of an excavator in the art to which the present invention pertains, and thus the detailed description of the configuration and operation thereof will be omitted to avoid redundancy.
Hereinafter, an operation example of the hybrid excavator will be described with reference to the accompanying drawings.
As shown in FIG. 1, as the hydraulic pump-motor 12 is rotated in a forward or reverse direction, a hydraulic fluid from the hydraulic pump-motor 12 is supplied to the large chamber 15b of the hydraulic cylinder 15 through the second flow path 14:14a; 14b, or a hydraulic fluid from the hydraulic pump-motor 12 is supplied to the small chamber 15a of the hydraulic cylinder 15 through the first flow path 13:13a; 13b so that the hydraulic cylinder 15 can be expanded or contracted.
As shown in FIG. 2, in a state in which a high pressure is generated in the large chamber 15b of the hydraulic cylinder 15 by a direction 1 of a load applied to the 10 hydraulic cylinder 15, the hydraulic fluid from the hydraulic pump-motor 12 is supplied to the large chamber 15b of the hydraulic cylinder 15 through the second flow path 14 in response to the drive of the electric motor 11, and the hydraulic fluid from the small chamber 15a of the 15 hydraulic cylinder 15 is drained through the first flow path 13 to cause the hydraulic cylinder 15 to be expanded.
A pressure formed in the second flow path 14 is higher than that formed in the first flow path 13, and thus the third hydraulic valve 21 using the hydraulic fluid of the 20 first and second flow paths 13 and 14 as a pilot signal pressure is shifted to the top on the drawing sheet. In this case, since the cross section of the large chamber 15b of the hydraulic cylinder 15 is larger than that of the small chamber 15a of the hydraulic cylinder 15, the hydraulic fluid compensated through a drain line 22 is supplied to the large chamber 15b of the hydraulic cylinder 15.
As shown in FIG. 3, in a state in which a high pressure is generated in the large chamber 15b of the hydraulic cylinder 15 by a direction 1 of a load applied to the 5 hydraulic cylinder 15, the hydraulic fluid from the hydraulic pump-motor 12 is supplied to the small chamber 15a of the hydraulic cylinder 15 through the first flow path 13 in response to the drive of the electric motor 11, and the hydraulic fluid from the large chamber 15b of the 10 hydraulic cylinder 15 is drained through the second flow path 14 to cause the hydraulic cylinder 15 to be contracted.
The high-pressure hydraulic fluid returned from the large chamber 15b of the hydraulic cylinder 15 is introduced into the hydraulic pump-motor 12 to cause the hydraulic 15 pump-motor 12 to generate electricity. A pressure formed in the second flow path 14 is higher than that formed in the first flow path 13, and thus the third hydraulic valve 21 is shifted to the top on the drawing sheet. In this case, since the cross section of the large chamber 15b of the 20 hydraulic cylinder 15 is larger than that of the small chamber 15a of the hydraulic cylinder 15, the hydraulic fluid compensated through a drain line 22 is supplied to the large chamber 15b of the hydraulic cylinder 15. At this time, since a flow rate of the hydraulic fluid discharged from the large chamber 15b of the hydraulic cylinder 15 is higher than that of the hydraulic fluid introduced into the small chamber 15a thereof, the hydraulic fluid flowing in the second flow path 14 is partially moved to the hydraulic tank T while passing through the connection 20 and the drain line 22.
As shown in FIG. 4, in a state in which a high pressure is generated in the small chamber 15a of the hydraulic cylinder 15 by a direction 2 of a load applied to the hydraulic cylinder 15, the hydraulic fluid from the hydraulic pump-motor 12 is supplied to the large chamber 15b of the hydraulic cylinder 15 through the second flow path 14 in response to the drive of the electric motor 11, and the hydraulic fluid from the small chamber 15a of the hydraulic cylinder 15 is drained through the first flow path 13 to cause the hydraulic cylinder 15 to be expanded. At this time, the high-pressure hydraulic fluid returned from the small chamber 15a of the hydraulic cylinder 15 is introduced into the hydraulic pump-motor 12 to cause the hydraulic pump-motor 12 to be driven to generate electricity.
A pressure formed in the first flow path 13 is higher than that formed in the second flow path 14, and thus the third hydraulic valve 21 is shifted to the bottom on the drawing sheet. Since a flow rate of the hydraulic fluid needed by the large chamber 15b of the hydraulic cylinder 15 is higher than that of the hydraulic fluid discharged from the small chamber 15a thereof. In this case, the hydraulic fluid from the hydraulic tank T is sucked in by the third hydraulic valve 21 through the drain line 22, and then joins the hydraulic fluid on the second flow path 14 through the first branch flow path 18.
As shown in FIG. 5, in a state in which a high pressure is generated in the small chamber 15a of the hydraulic cylinder 15 by a direction 2 of a load applied to the hydraulic cylinder 15, the hydraulic fluid from the hydraulic pump-motor 12 is supplied to the small chamber 15a of the hydraulic cylinder 15 through the first flow path 13 in response to the drive of the electric motor 11, and the hydraulic fluid from the large chamber 15b of the hydraulic cylinder 15 is drained through the second flow path 14 to cause the hydraulic cylinder 15 to be contracted.
A pressure formed in the first flow path 13 is higher than that formed in the second flow path 14, and thus the third hydraulic valve 21 is shifted to the bottom on the drawing sheet. Since a flow rate of the hydraulic fluid discharged from the large chamber 15b of the hydraulic cylinder 15 is higher than that of the hydraulic fluid introduced into the hydraulic pump-motor 12. In this case, the hydraulic fluid flowing in the second flow path 14 is partially moved to the hydraulic tank T through the first branch flow path 18, the third hydraulic valve 21, and the drain line 22.
As shown in FIG. 6, in the case where the operation of the machine is stopped in a position of an attachment 6 consisting of the boom 1 and the like, a low load occurs in the above-mentioned load direction 1 (e.g., the case where the hydraulic cylinder is contracted) in the respective hydraulic cylinders 15, 4 and 5. In this case, the first and second hydraulic valves 16 and 17 are shifted to a position in which the first and second flow paths 13 and 14 are closed in order to prevent the hydraulic fluid from leaking to the outside when the hydraulic cylinders are not driven, and thus the internal pressure of the hydraulic cylinders is not dropped.
In the meantime, since the hydraulic fluid has somewhat compressibility, vibration may occur due to the abrupt stop of the attachment 6 or the operation (e.g., the case where the drive of the boom cylinder 15 is stopped while the arm cylinder 4 is driven) of another hydraulic cylinder.
As shown in FIG. 7, even in the case where the first and second hydraulic valves 16 and 17 are closed, the hydraulic fluid of the hydraulic cylinder 15 is compensated so that a constant pressure is generated even after occurrence of the vibration. The cross section of the large chamber 15b of the hydraulic cylinder 15 is larger than that of the small chamber 15a thereof (e.g., twice larger than that of the small chamber 15a in a general excavator). Thus, even in the case where the same pressure is generated in the large and small chambers, a force allowing the piston to be moved in the large chamber 15b is larger than in the small chamber 15a. When a pressure of the large chamber 15b is a half that of the small chamber 15a, the forces of the large chamber 15b and the small chamber 15a, which push each other, become the same. In the case where the boom cylinder 15 is contracted by the load direction 1, a pressure (a) of the small chamber 15a is higher than a pressure (b) of the large chamber 15b (see FIGS. 7 and 8).
As shown FIGS. 8 and 9, the first and second hydraulic valves 16 and 17 are shifted to an opened position through 15 the application of a control signal thereto to perform a work under the conditions where an external force is applied to the hydraulic cylinder 15 by the load direction 1, so that a high pressure is formed in the first flow path 13 and a low pressure is formed in the second flow path 14 to 20 cause the third hydraulic valve 21 to be shifted to the bottom on the drawing sheet.
As shown in FIGS. 9 and 10, when the pressure formed in the large chamber 15b is released while the piston of the hydraulic cylinder 15 is moved by several millimeters (mm), the third hydraulic valve 21 is shifted to the top on the drawing sheet to cause the hydraulic cylinder 15 to be operated normally.
As shown in FIGS. 8 and 9, in the process in which the first and second hydraulic valves 16 and 17 are shifted to an opened position from a closed position, and the third hydraulic valve 21 in a neutral position is shifted to the bottom on the drawing sheet by the pressure of the first flow path 13, the piston of the hydraulic cylinder 15 is moved by several millimeters (mm). In this case, although the movement distance of the piston of the hydraulic cylinder 15 is not long, a distal end of the attachment 6 is moved by several meters (m), thereby causing a problem in that manipulability and workability are deteriorated.